A thermometric bimetallic structure consists generally of two joined plates or strips of metals having different coefficients of expansion so that a temperature rise causes the bimetallic structure to change its shape in dependence on temperature.
This property is utilized in engineering in many cases for automatic control by temperature of other physical quantities which are related to temperature, such as the electric current, e.g., in electric motors, in order to prevent an overloading thereof.
The coefficient of excursion (deflection) of a thermometric bimetal from an original position depends essentially on the physical properties of the joined metals and on the dimensions of the temperature-sensing and switching elements made therefrom. For this reason the accuracy of the operation of such switching elements depends on the quality of the component metals and on the precision with which they have been joined.
In general, the highest coefficients of excursion, e.g. of an automatic control element, will be obtained if the so-called active component has a high thermal coefficient of expansion and the passive component has a low thermal coefficient of expansion. The excursion as such is known to depend on the temperature responses of the coefficients of expansion of the two components of the bimetal.
The dependence of the mechanical strength of the components on temperature is also important because this dependence often determines the upper limit of the temperature range in which the bimetallic structure may be used.
The previously known thermometric bimetallic structure includes combinations that have been developed for use up to a very high upper temperature limit. The bimetallic structures which are presently available on the market can only be used up to an upper temperature limit of about 500.degree. C, because above this temperature the coefficients of expansion of the iron-nickel alloys used as passive components increase so sharply that the laminated bimetallic structure no longer responds to a further temperature rise. Additionally one component or both components can soften at temperatures above 500.degree. C so that the temperature rise results in a permanent deformation of the bimetallic structure and the latter does not return to its original shape when cooled.
Owing to the low strength of the component or both components at elevated temperatures, the bimetallic structure can exert only small actuating or control forces and for this reason cannot perform the desired switching operation in many cases.
On the other hand, there is a general desire to provide automatic and other control systems for use at higher temperatures above 500.degree. C.
It has been found that the thermometric bimetallic structures which have been available to date do not meet the requirements or do not sufficiently meet the requirements. This remark is applicable, e.g., to widely used domestic appliances, such as toasters, or to motor vehicle exhaust systems providing for a decontamination of exhaust gases.